In digital computer systems, it is common practice to use the exclusive or (XOR) relationship between corresponding bits of storage data to provide a foundation for detecting and correcting data errors when the stored data is retrieved. Typically, the XOR data is written in synchronization with corresponding written storage data from which it is generated. The XOR data is generally produced from a plurality of storage bits. These storage bits are simultaneously stored on a plurality of different data storage channels. These data storage channels may comprise different tracks of a single storage medium, or may comprise a plurality of completely separate, but synchronized, recording media. The XOR data is generated by reading the stored data and then making bit-by-bit comparisons of the stored data. The resulting XOR data is then recorded on a separate data storage channel in synchronization with the corresponding data bits on the data storage channels from which the XOR data is computed.
However, because of the computational delay required to produce the XOR data, the XOR data cannot be synchronously written on the same write cycle of the data storage medium as the corresponding written data bits. In the case of data storage discs, the XOR data must be written during a subsequent revolution of the disc in the same sector from which the stored data that was used to generate the XOR data is located. In this manner the XOR data is synchronized with the corresponding storage data from which it was generated. Consequently, the entire duration of the write process is the sum of the interval required for the written data, plus the "XOR latency", which is the interval required for XOR data computation and writing. The XOR latency in this case is the interval corresponding to a complete memory write cycle, such as a complete revolution on a successive pass of a data storage disc, so that the XOR data is properly synchronized on the disc.
For instance, for a particular data storage disc drive, the disc sector write interval may be 238 microseconds, and the interval for a complete pass, or revolution, of the disc may be 16.6 milliseconds. If a four sector write operation is performed, the total duration of the write process is 952 microseconds. When a single disc drive is so used, data storage integrity is sufficiently high to dispense with the XOR data. Thus, the total duration of the write process remains 952 microseconds. Of course, the total duration of the read process for this same written data is also 952 microseconds.
If the data to be written is spread over four disc drives, the four sector write operation is reduced to a single sector write operation. The decrease in reliability of data stored and retrieved with this multiple disc write operation mandates some form of data integrity enhancement, such as the XOR data generation described above. When the XOR data is used, an additional disc drive, synchronized with the other four, records the XOR data. However, the interval required to compute the XOR data prevents it from being recorded in synchronization with the corresponding data from which it is generated on the same pass of the synchronized discs. Instead, the XOR data is held until the next pass of the XOR data disc, and then the XOR data is written in synchronization with the corresponding data associated with the XOR data. As indicated above, the interval required for a complete revolution of the XOR disc in this case is 16.6 milliseconds. Therefore, the total write time for this scheme is 238 microseconds plus 16.6 milliseconds, or 16,838 microseconds. Of course, the total read time is considerably less, being 238 microseconds plus the XOR computational latency, which is a nominal value dependent upon the system hardware used.
Thus, it is apparent that a multiple data storage medium configuration, such as described above, can greatly improve system data rates for the read process, although the write process is considerably lengthened.